The invention refers to devices and methods for the introduction of ions into special Kingdon ion traps in which the ions can harmonically oscillate in the longitudinal direction, completely decoupled from their transverse motion. Kingdon traps are electrostatic ion traps in which ions can orbit around one or more inner electrodes or oscillate between several inner electrodes, while an outer, enclosing housing is at a DC potential which the ions with a predetermined total energy cannot reach. In special Kingdon traps particularly suited as mass spectrometers, the inner surfaces of the housing electrodes and the outer surfaces of the inner electrodes are designed so that, firstly, the motions of the ions in the longitudinal direction of the Kingdon trap are completely decoupled from their motions in the transverse direction and, secondly, a parabolic potential profile is generated in the longitudinal direction in which the ions can oscillate harmonically.
In this disclosure, the term “Kingdon trap” refers only to these special forms in which ions can oscillate harmonically in the longitudinal direction, completely decoupled from their transverse motions. These Kingdon ion traps can be used as mass spectrometers by measuring the oscillation frequencies of ions in longitudinal direction.
Kingdon traps must be operated under ultra-high vacuum if ions are to be stored undisturbed for a prolonged time to measure their oscillations. During this time the ions must not suffer any collisions with the residual gas because they would then lose kinetic energy and finally hit the inner electrode arrangement.
If packets of ions move coherently in the longitudinal direction in the parabolic potential profile, the ion packets with different charge-related mass each oscillate with their own, mass-dependent frequencies. The frequencies are inversely proportional to the square root √(m/z) of the charge-related mass m/z. The image currents induced by the oscillating ions can be measured in the form of a time-dependent image current transient in suitable detection electrodes, for example in two half electrodes of a housing which is centrally split. A Fourier analysis of this image current transient produces a frequency spectrum; and the frequency spectrum is then transformed into a mass spectrum. As is the case with other Fourier transform mass spectrometers, such as ion cyclotron resonance mass spectrometers, a very high mass resolution R can be achieved by long measurement times. The precondition is that the shape of the outer and inner electrodes is very precisely manufactured because the harmonic potential profile depends on the shape of these electrodes.
The advantage of Kingdon trap mass spectrometers compared to ion cyclotron resonance mass spectrometers (ICR-MS) with similarly high mass resolutions R consists in the fact that no superconducting magnet is required to store the ions, and so the technical set-up is much less costly. The ions are stored here oscillating or orbiting in a DC field and thus require only DC voltages at the electrodes, although these DC voltages must be kept constant with a very high degree of precision. Moreover, the decrease in resolution R with mass in Kingdon trap mass spectrometers is only inversely proportional to the square root √(m/z) of the charge-related mass of the ions m/z, whereas in ICR-MS the decrease in resolution R is inversely proportional to the mass m/z itself; this means the resolution falls off much more rapidly toward higher masses in ICR-MS.
The patent specification U.S. Pat. No. 5,886,346 (A. A. Makarov) elucidates the fundamentals of a special Kingdon trap which has been marketed by Thermo-Fischer Scientific GmbH Bremen under the name Orbitrap®. The Orbitrap® consists of a single spindle-shaped inner electrode and coaxial housing electrodes transversely split down the center. The housing electrodes have an ion-repelling and the inner electrode an ion-attracting electric potential. With the aid of an ion-optical device, the ions are tangentially injected as ion packets through a hole in the housing electrode and orbit in a hyperlogarithmic electric potential. The kinetic injection energy of the ions is adjusted so that the attractive forces and the centrifugal forces balance each other out, and the ions therefore largely move on almost circular trajectories.
The electric potential of the Orbitrap® has a parabolic potential well in the longitudinal direction, in which the transversely orbiting ions can oscillate harmonically in the longitudinal direction. The ion packets oscillating in the longitudinal direction induce image currents in the hemispherical electrodes of the centrally split housing, and these currents are measured in the form of the image current transient as a function of time. As has been described above, mass spectra can be obtained from these image current transients. The mass resolution of an Orbitrap® is currently around R=50,000 at m/z=1,000 daltons, and even higher for good instruments. The electrodes must be manufactured to a very high degree of mechanical precision. In addition, the introduction (injection) of the ions is critical because the kinetic energy of the ions on injection must only vary within a small tolerance range. The injection technique is complicated and requires that the operating voltage between the outer and inner electrodes be continuously increased during the injection process. This requirement to change the operating voltage is disadvantageous because this operating voltage must remain exceptionally constant, preferably to better than one millionth of its value, during the measurement of the image currents in order to achieve high mass accuracies. The requirement to keep a changeable voltage constant poses special electro-technical problems.
The patent application DE 10 2007 024 858.1 (C. Köster) describes further types of Kingdon trap with several inner electrodes which differ in the way these electrodes are arranged. In this case, as well, the inner electrodes and the outer housing electrodes can be precisely formed in such a way that the longitudinal motion is completely decoupled from the transverse motion and a parabolic potential well is created in the longitudinal direction for a harmonic oscillation. The patent application cited contains the mathematical expressions for the equipotential surfaces inside such Kingdon traps, which also describe the exact outer shapes of the inner electrodes and inner shapes of the housing electrodes, because each of these must form equipotential surfaces of the desired field. The embodiments listed also include those where the analyte ions can oscillate transversely in the center plane between at least one pair of inner electrodes, practically in a single plane. The analyte ions oscillating transversely in this way can then execute harmonic oscillations in the longitudinal direction, and the resulting image currents can then be measured in order to produce the high-resolution mass spectrum.
For all Kingdon traps it is advantageous to introduce the ions at a location in the longitudinal direction outside the potential minimum. The introduced ions then immediately start to oscillate not only in the transverse, but also in the longitudinal direction without having to be particularly excited to these oscillations. The exact location at the outer electrode where the ions are introduced then marks the reversal points of the longitudinal oscillations. No special voltage generator is thus required for the excitation of these oscillations, which means no generator for “chirp” or “synch pulses”, as is required for the excitation of the ions in ICR mass spectrometers.
Mass spectrometers can only ever determine the ratio m/z of the ion mass m to the number z of elementary charges of the ion. In the following, the term “mass of an ion” or “ion mass” always refers to the ratio m/z of the mass m to the number z of positive or negative elementary charges of the ion, that is to say the elementary charge-related (“charge-related” for short) mass m/z. Among the key criteria influencing the quality of a mass spectrometer are mass resolution and the mass accuracy. Mass resolution is defined as R=(m/z)/Δ(m/z)=m/Δm, where R is the resolution, m the mass of an ion measured in units of the mass scale, and Δm the width of the mass signal at half maximum, measured in the same units. The term mass accuracy relates to both the statistical spread about a measured mean value and the systematic deviation of the measured mean value from the true value of the mass; the latter can be made to disappear by exact calibration.